The present invention relates to a Faraday rotator for use with high-energy laser light.
Recently, optical fiber communications and optical instrumentation have taken a giant leap forward. Semiconductor lasers and solid state lasers are most commonly used light sources for optical fiber communications and optical instrumentation. Lasers have a serious disadvantage that the oscillation becomes unstable if there is reflect light return, i.e., the laser light is reflected by, for example, the end surface of the optical fiber and returns to the laser element. In order to solve this problem, an optical isolator is provided at the exit of the laser light, thereby blocking the reflect light return to stabilize oscillation of the laser source.
Optical isolators are usually constructed of a polarizer, an analyzer, a Faraday rotator, and a permanent magnet that causes the Faraday rotator to be magnetically saturated. The Faraday rotator plays a critical role in the optical isolator and takes the form of a bismuth-substituted rare-earth iron garnet single crystal (referred to as BIG hereinafter) film having a thickness in the range from several tens of microns to 450 microns, grown by the liquid phase epitaxial (referred to as LPE herinafter) method. Such bismuth-substituted iron garnet single crystals include (HoTbBi)3Fe5O12 and (TbBi)3 (FeAlGa)5O12.
A BIG is usually grown by the LPE method as follows:
A crucible made of a noble metal is placed in the middle of an LPE apparatus in the form of a vertical furnace. Iron garnet compositions such as ferric oxides and oxides of rare-earth elements are introduced into the crucible together with flux materials including lead oxide, boron oxide, and bismuth oxide.
The oxides are heated to about 1000xc2x0 C. to provide a melt, which is subsequently used to grow a BIG. Then, the melt is cooled to about 800xc2x0 C. so that the melt is super-saturated. Then, a non-magnetic garnet substrate is attached to a substrate holder and slowly lowered from the upper part of the LPE furnace until the non-magnetic garnet substrate comes into contact with the surface of the melt. The substrate is then rotated in contact with the melt so that a garnet single crystal is epitaxially grown on the undersurface of the substrate. After the garnet single crystal having a desired thickness has grown, the substrate is lifted several centimeters above the surface of the melt and is spun at a high speed to spin-remove most of the melt adhering to the substrate. Then, the substrate is taken out from the LPE furnace.
The BIG obtained in this manner is subjected to the polishing process to separate the substrate from the BIG. During the polishing process, the BIG is polished to a desired thickness. The substrate is separated from the as-grown BIG, to obtain as thin a Faraday rotator as possible and to eliminate Fresnel reflection that occurs in the interface between the substrate and the BIG so that the transmittance of the Faraday rotator is as high as possible. Then, anti-reflection films are formed on both sides of the crystal by vacuum vapor deposition. Finally, using a dicing machine or a scriber, the crystal film is cut into small sizes of, for example, 1.5 mmxc3x971.5 mm.
The BIG is excellently transparent to light in the near infrared region but has large light absorption depending on wavelengths. For example, if the Faraday rotation of the BIG is 45 degree, then the BIG has a loss of about 1 dB at room temperature for light having a wavelength of 1,064 nm generated by a YAG laser using Y3Al5O12. The energy of light absorbed by the BIG is converted into heat, which in turn increases the temperature of the BIG.
If the laser power is low, for example, less than 1 mW, the temperature rise of the BIG is negligible.
However, signal light for optical fiber communications in the 1,550-nm range, generated by the YAG laser or excited by an optical fiber amplifier referred to as erbium doped fiber amplifier (abbreviated as EDFA), often has a power of several hundred milliwatts. When such high-power light is incident on the BIG, the temperature of the BIG may increase significantly so that the temperature rise is no longer negligible.
The temperature rise of BIG causes some problems. For example, the isolation of an optical isolator decreases. The Faraday rotation of a Faraday rotator is temperature dependent. Thus, the Faraday rotation varies with increasing temperature of the BIG, causing changes in the isolation performance of the optical isolator.
Another problem is that the insertion loss increases since the light absorption coefficient of the BIG increases with increasing temperature. If the heat generated in the BIG is not dissipated sufficiently, the temperature of the BIG increases extremely so that the antireflection film may become detached or the BIG may become damaged. Thus, the BIG cannot be applied as a Faraday rotator if the laser in use has very high power.
An object of the invention is to develop BIG that can be used as a Faraday rotator for use with high power laser light.
A high-energy laser Faraday rotator is used in an optical system in which the Faraday rotator absorbs an amount of laser light energy more than 20 mW. The Faraday rotator includes a bismuth-substituted rare-earth iron garnet single crystal film grown as a Faraday rotator by using a liquid phase epitaxial method, and non-magnetic substrate having a side surface on which the bismuth-substituted rare-earth iron garnet single crystal film is grown.
The Faraday rotator absorbs an amount of energy of laser light in the range from 20 to 600 mW.
The non-magnetic garnet substrate has a thickness in the range from 0.2 to 0.8 mm.
An optical isolator may incorporate the high-energy laser Faraday rotator of the invention, wherein the Faraday rotator is assembled with the BIG film positioned on the light incidence side (high energy laser side) and the non-magnetic substrate on the light exiting side (remote from the high energy laser).
The optical isolator includes a polarizer, a Faraday rotator, and an analyzer, all being aligned in this order from a laser side, and adjacent elements being assembled in intimate contact with each other.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.